In recent years, a motor has often employed a method in which a motor is driven by an inverter using a pulse width modulation system (to be arbitrarily referred to as a PWM system hereinafter). In the inverter driving of the PWM system, since a neutral-point potential of a winding is not zero, a potential difference (to be referred to as a shaft voltage hereinafter) is generated between an outer ring and an inner ring of a bearing. The shaft voltage includes a high-frequency component obtained by switching. When the shaft voltage reaches a breakdown voltage of an oil film in the bearing, a minute electric current flows in the bearing to cause electric corrosion in the bearing. When the electric corrosion progresses, a wavelike abrasion phenomenon may occur on a bearing inner ring, a bearing outer ring, or bearing balls to generate abnormal sound, and the abnormal sound is one main factor of drawback in the motor.
A power supply circuit of a drive circuit (including a control circuit or the like) that inverter-drives a motor by a PWM system, a primary circuit of the power supply circuit, and the earth on the primary circuit side are electrically insulated from each other.
Conventionally, in order to suppress electric corrosion, the following measure is conceived.
(1) The bearing inner ring and the bearing outer ring are electrically conductive to each other.
(2) The bearing inner ring and the bearing outer ring are electrically insulated from each other.
(3) A shaft voltage is reduced.
As a concrete method for the (1), a method of giving conductivity to a lubricant agent of the bearing is given. However, a conductive lubricant agent is deteriorated in conductivity with elapsed time, and lacks in sliding reliability. A method of arranging a brush on a rotating shaft to set a conductive state may be conceived. However, this method disadvantageously requires brush abrasion powder or a space.
As a concrete method of the (2), a method of changing iron balls in the bearing with nonconducting ceramic balls is given. This method highly effectively suppresses electric corrosion but disadvantageously has high cost. The method cannot be employed in a general-purpose motor.
As a concrete method of the (3), a method of electrically short-circuiting a stator iron core to a conductive metal bracket to change a capacitance to reduce shaft voltage is conventionally known (for example, see Patent Document 1). As conventional techniques that suppress electric corrosion of bearings of motors, a large number of configurations in each of which a stator iron core or the like of a motor is electrically connected to the earth are disclosed.
An impedance obtained when an electric capacity and a resistor element are connected in parallel with each other is expressed by a relational expression: Z=1/jωC+R. In this expression, Z denotes an impedance, j denotes an imaginary number, ω denotes an angular frequency, C denotes a capacitance, and R denotes a resistance. As is apparent from expression, when the capacitance increases or when the resistance decreases, the impedance decreases. In contrast to this, when the capacitance decreases or when the resistance increases, the impedance increases.
In Patent Document 1, the stator iron core and the bracket are short-circuited to decrease an impedance on the stator side. In this manner, the bearing is suppressed from being electrically corroded.
More specifically, in general, a motor that is used in a washing machine, a dishwasher, or the like near water and that may electrocute a person needs not only to have a charging unit insulated (basic insulation) but also to be additionally independently insulated (to be referred to as additional insulation hereinafter). On the other hand, since a motor that is used in an indoor unit or an outdoor unit of an air conditioner, a water heater, an air purifier, or the like in addition to a washing machine, a dishwasher, or the like may not electrocute a person, the motor does not require additional insulation. Therefore, since the motor that is used in an indoor unit or an outdoor unit of an air conditioner, a water heater, an air purifier, or the like does not have an insulated rotor, an impedance on the rotor side (bearing inner ring side) is at a low level. In contrast to this, since the stator side (bearing outer ring side) has an insulated structure, an impedance is at a high level. In this case, voltage drops caused by impedances are different from each other. Since a potential on the bearing inner ring side and a potential on the bearing outer ring side are high and low, respectively, an imbalanced state occurs, and a high shaft voltage is generated. The high shaft voltage may cause electric corrosion in the bearing.
In order to avoid the state, Patent Document 1 employs a method that short-circuits the stator iron core and the bracket to each other to eliminate a capacitance component therebetween and, as described above, decreases the impedance on the stator side (bearing outer ring side) to approximate the impedance to the impedance on the rotor side (bearing inner ring side).
In recent years, a molded motor the reliability of which is improved by molding a fixing member such as a stator iron core on a stator side with a molding material is proposed. For this reason, a bearing may be fixed with the insulating molding material in place of a metal bracket to suppress an unnecessary high-frequency voltage generated on the bearing outer ring side and an unnecessary high-frequency current flowing between the inner ring and the outer ring of the bearing. However, the molding material is a resin that is not strong enough to fix the bearing and has poor dimensional accuracy for resin molding, and creeping in the bearing disadvantageously easily occurs. More specifically, in a shaft bearing such as a bearing, in general, for example, when a void is formed between an outer ring and a housing inner circumferential surface, a force in a radial direction is generated on a shaft by a transmission load. When such a force is generated, a slipping phenomenon is easily generated by a relative difference in a radial direction. The sleeping phenomenon is called creeping. The creeping, in general, can be suppressed by strongly fixing the outer ring to a housing such as a bracket. In recent years, with the progress of a recent high-power motor, a bearing needs to be more strongly fixed. For this reason, for example, as a countermeasure against creeping, a metal bracket processed with a steel plate in advance and having preferable dimensional accuracy must be employed to fix a bearing. Especially, a general bearing has a structure in which a rotating shaft is supported at two positions. However, in terms of the strength described here, because of the easiness of execution and the like, the shaft is preferably fixed to two bearings with metal brackets.
However, the conventional method as described in Patent Document 1 has the following problem. More specifically, since the conventional method is a method using short-circuiting, an impedance cannot be adjusted, and a shaft voltage may increase depending on a magnet material or a structure of a rotor. More specifically, by the short-circuiting serving as the conventional method, an impedance on the stator side decreases considerably more than the impedance on the rotor side. In this manner, a high shaft voltage may be generated.
Because of the strength described above, when the two bearings are fixed with two metal brackets, in general, shapes and arrangement states of one bracket and the other bracket are different from each other. For this reason, the impedances of both the brackets are different from each other. For this reason, a potential induced by one bracket is different from a potential induced by the other bracket. For this reason, the shaft voltages of the two bearings are also different from each other. Even though electric corrosion does not occur in one bearing, electric corrosion may disadvantageously occur in the other bearing.
In the configuration in the problem of the application, as described above, a power supply circuit of a drive circuit (including a control circuit or the like) that inverter-drives a motor by a PWM system, a primary circuit of the power supply circuit, and the earth on the primary circuit side are electrically insulated from each other. Therefore, it is difficult to employ a conventional configuration in which a stator iron core or the like of a motor is electrically connected to the earth and to solve the problem by a configuration added with the conventional technique because still another problem is posed in terms of specifications and characteristics of a motor.    Patent Document 1: Unexamined Japanese Patent Publication No. 2007-159302